The present invention relates to a digital cylinder-to-plate type lithographic printing method and a lithographic printing apparatus, and particularly to a method of plate making with oil-based ink, printing with such plates and printing apparatuses characterized by superior image quality of the plate as well as the final printed matter.
In the conventional lithographic printing, an ink-receptive area and an ink-repelling area are formed on the surface of a printing plate, and a printing ink is fed on the plate so as for the ink to selectively adhere to the ink-receptive area. The adhering printing ink is then transferred to paper. Usually, the hydrophilic area and the oleophilic (ink-receptive) area are formed imagewise on the surface of a printing plate. Then, the hydrophilic area is moistened with dampening water to repel the printing ink.
Image recording on the printing plate material (plate making) is carried out, as the most popular method, by first outputting, via an analog or a digital method, an original image on a silver halide photographic film, through which a photosensitive diazo resin or a photopolymer-based layer is exposed to light, and removing such a photosensitive layer at the non-image areas with an alkaline developer.
Recently, with the advance of digital image formation technologies and with the demand for a higher efficiency of printing workflow, a variety of proposals are being made on a system that can directly output images on printing plate using digital image information. Such methods are often called CTP (Computer-To-Plate), or DDPP (Digital Direct Printing Plate). The plate making method suited for CTP includes those based on laser exposure in light or heat mode, and some of them are being in practical use.
However, such plate making methods based on laser exposure suffer from an environmental drawback caused by the use of alkaline developer needed to remove background areas of the plate material after image exposure. This drawback is common to the light and heat modes.
In order to make printing process efficient, systems are proposed in which plate making is carried out on printing apparatuses. Some of such systems are based on laser exposure, but they require expensive and bulky apparatuses. Hence, systems based on inkjet imaging are under investigation as they use inexpensive and compact image recording apparatuses.
Japanese Patent Laid-Open No. 97848/1992 discloses such an on-cylinder image recording system in which a plate drum having a hydrophilic or an oleophilic surface is used instead of the conventional plate cylinder, and in which an oleophilic or a hydrophilic image is formed with inkjet recording. The image is then used for printing, and removed or erased after printing. However, this method suffers from a difficulty in the consistency of the ease of image erasing with image durability. Further, in order to form sufficiently durable images on the plate cylinder, inkjet inks with relatively high contents of resinous ingredients concentrations are required. Such type of ink tends to cause the solidification of the resin at inkjet nozzles due to solvent evaporation there, leading to a poor consistency in ink ejection. Thus, it is difficult to consistently form high quality images.
Japanese Patent Laid-Open No. 27953/1989 discloses a plate making method comprising image formation by inkjet recording using an oleophilic wax ink onto a hydrophilic plate material. However, the wax image made by this method suffers from a poor print durability because waxes are mechanically weak and poorly adhere to the hydrophilic plate surface.
Japanese Patent Laid-Open No.268227/1999 discloses a computer-to-cylinder type printing method in which image recording is carried out by an inkjet recording process. The process comprises application of an intense electric field at an ink ejecting point to ink comprising a hydrophobic particulate resinous material dispersed in an insulating solvent so as for the resinous material to aggregate and eject as a highly condensed fluid. Owing to such concentration mechanism, dots formed by this method have a sufficient thickness enough to stand large run lengths. However, in this electrostatic inkjet recording, ink ejects under the application of a potential as high as several kV, and in cases where the recording head has a plurality of ejecting channels, adjacent channels tend to suffer from electric field interference that makes the flying locus of ejected ink droplets unstable, leading to inaccurate dot placement on the recording plane. Therefore, the electric field interference makes dense arrangements of the ejecting channels difficult.
The present invention has been made to solve the foregoing problems.
Accordingly, an object of the invention is to provide a lithographic printing method and apparatus not requiring any development processing in which the electric field interference among the ejecting channels of the recording head is prevented.
Another object of the invention is to provide a lithographic printing method and apparatus is capable of making, via inexpensive and simple methods, a printing plate from which a large number of high quality prints can be produced.
Other objects and effects of the invention will become apparent from the following description.
The above-described objects of the invention have been achieved by providing the following items.
(1) A method of computer-to-cylinder lithographic printing, comprising:
loading a plate material on a rotative plate cylinder of a lithographic printing apparatus;
rotating said plate cylinder having loaded thereon the plate material;
forming an image directly onto the plate material by an inkjet image-recording process which comprises ejecting an oil-based ink from a recording head having a plurality of ejecting channels, based on image data signals, utilizing an electrostatic field, to prepare a printing plate;
subsequently performing lithographic printing with the thus prepared printing plate,
wherein said recording head is driven so that every n""th channel thereof is actuated in a common phase, and
wherein said plate cylinder is rotated to give a surface rotational speed V (mm/sec) of the plate material as represented by the following formula:
V=25.4xc3x97(fxc3x97n)/N
wherein N represents a recording resolution (dots/25.4 mm) along a rotative direction of the plate cylinder on said plate material, and f represents a driving frequency f (Hz) of each ejecting channel of said recording head.
(2) The computer-to-cylinder lithographic printing method according to item (1) above, wherein said oil-based ink comprises;
a non-aqueous solvent having a specific resistance not lower than 109 xcexa9cm and a dielectric constant not higher than 3.5; and
a hydrophobic particulate resin dispersed in said solvent, the resin being solid at least at room temperature.
(3) A computer-to-cylinder lithographic printing apparatus comprising:
a rotative plate cylinder on which a plate material is to be loaded;
an image forming unit comprising an inkjet recording unit including a recording head having a plurality of ejecting channels so as to form an image directly on the plate material loaded on said plate cylinder by ejecting an oil-based ink from said recording head, based on image data signals, utilizing an electrostatic field to prepare a printing plate;
an image data processing and control unit which drives said recording head so that every n""th channel of said recording head is actuated in a common phase;
a plate cylinder""s rotational speed-controlling unit which control the rotational speed of said plate cylinder to give a surface rotational speed V (mm/sec) of the plate material as represented by the following formula:
V=25.4xc3x97(fxc3x97n)/N
wherein N represents a recording resolution (dots/25.4 mm) along a rotative direction of the plate cylinder on said plate material, and f represents a driving frequency f (Hz) of each ejecting channel of said recording head; and
a lithographic printing unit which performs lithographic printing with the thus prepared printing plate.
(4) The computer-to-cylinder lithographic printing apparatus according to item (3) above, wherein said oil-based ink comprises:
a non-aqueous solvent having a specific resistance not lower than 109 xcexa9cm and a dielectric constant not higher than 3.5; and
a hydrophobic particulate resin dispersed in said solvent, the resin being solid at least at room temperature.
(5) The computer-to-cylinder lithographic printing apparatus according to item (3) or (4) above, wherein said image forming unit further comprises an ink fixing unit.
(6) The computer-to-cylinder lithographic printing apparatus according to any one of items (3) to (5), wherein said image forming unit further comprises a dust cleaning unit which removes dust present on the plate at least one of prior to and during image recording onto said plate material.
(7) The computer-to-cylinder lithographic printing apparatus according to any one of items (3) to (6) above, wherein said image forming unit rotates said plate cylinders to perform main scanning upon image recording onto the plate material.
(8) The computer-to-cylinder lithographic printing apparatus according to item (7) above, wherein said recording head comprises multiple channels and is movable along a direction parallel to an axis of said plate cylinder to perform sub-scanning upon image recording onto the plate material.
(9) The computer-to-cylinder lithographic printing apparatus according to item (7) above, wherein said recoding head comprises a full-line head having a width substantially equal to that of said plate material.
(10) The computer-to-cylinder lithographic printing apparatus according to any one of items (3) to (9) above, wherein said inkjet recording unit further comprises an ink feeding member which feeds the ink to said ink ejecting head.
(11) The computer-to-cylinder lithographic printing apparatus according to item (10) above, wherein said inkjet recording unit further comprises an ink recovery member which recovers said oil-based ink from said recording head to circulate said ink in cooperation with said ink feeding member.
(12) The computer-to-cylinder lithographic printing apparatus according to any one of items (3) to (11) above, wherein said inkjet recording unit further comprises an ink tank and an ink agitating member installed inside said ink tank.
(13) The computer-to-cylinder lithographic printing apparatus according to item (12) above, wherein said inkjet recording unit further comprises an ink temperature control member installed inside said ink tank.
(14) The computer-to-cylinder lithographic printing apparatus according to any one of items (3) to (13) above, wherein said inkjet recording unit further comprises an ink concentration control member.
(15) The computer-to-cylinder lithographic printing apparatus according to any one of items (3) to (14) above, wherein said inkjet recording unit further comprises a recording head distancing/approximating member capable of approximating said recording head to said plate cylinder upon image recording onto the plate material and of distancing said recording head from said plate cylinder except during the image recording.
(16) The computer-to-cylinder lithographic printing apparatus according to any one of items (3) to (15) above, wherein said image forming unit further comprises a cleaning member which cleans said ink ejecting head at least after the completion of the plate making.
(17) The computer-to-cylinder lithographic printing apparatus according to any one of items (3) to (16) above, wherein said lithographic printing unit comprises a dust removing member which removes paper dust generating during lithographic printing.
(18) The computer-to-cylinder lithographic printing apparatus according to any one of items (3) to (17) above, wherein said image forming unit has a recording head temperature control member.